Powder/granular material feeding devices, also called feeders, have been widely known as devices for quantitative feeding of powder/granular material. As illustrated in FIGS. 20 and 21, such a powder/granular material feeding device includes a hopper 51 that receives powder/granular material, an outlet portion 52 having screws 52a for discharging powder/granular material, a chute 53 that guides, into the outlet portion 52, powder/granular material dropped from the hopper 51, and a stirring device 54 that is also called an agitator for agitating powder/granular material in the chute 53 and the hopper 51. The powder/granular material feeding device for quantitative feeding of powder/granular material includes, in addition to these constituent elements (the hopper 51, the outlet portion 52, the chute 53, and the stirring device 54), a metering unit 55 on which these constituent elements are loaded, and a control unit (not shown) that controls the outlet portion 52 so as to keep constant the weight of discharged powder/granular material (the amount of discharged powder/granular material) per hour or the weight of powder/granular material in each discharge.
Various kinds of powder/granular material feeding devices are available. The hopper 51 in FIGS. 20 and 21 is a cylinder that is vertically extended with a constant shape in cross section. A hopper 82 in FIGS. 22, 23A, and 23B is shaped like a rectangular prism. A hopper 61 in FIG. 24 has a curved shape, e.g., a conical or pyramidal shape expanding upward in cross section. As simply illustrated in FIG. 24, the curved hopper 61 expands upward in cross section and can be advantageously reduced in height with a sufficient capacity. However, the hopper 61 with a curved wall surface decreases in cross-sectional area toward the bottom of the hopper 61, causing bridging, that is, partially or entirely remaining powder/granular material with high adhesion and compressibility or a rathole that is a central hole of a material. These phenomena are effectively avoided by adopting a cylindrical hopper or hanging, as illustrated in FIG. 24, a vertical agitator (stirring part) 62 downward into the hopper 61 from the top surface of the hopper 61. A hopper agitator including such a vertical agitator is disclosed in, for example, Japanese Patent Laid-Open No. 9-216688.
Unfortunately, the provision of the vertical agitator 62 requires a space for lifting the agitator 62 during cleaning of the hopper 61 (the hopper 61 is always cleaned each time a powder/granular material type is changed). Furthermore, a top surface 61a of the hopper 61 needs to be lifted with an additional weight of the agitator 62 and an electric motor 63 for driving the agitator, reducing workability of cleaning and so on. For this reason, the agitator in general is not so popular among users.
The chute provided under the hopper contains the stirring device that is a mechanism for agitating powder/granular material in the hopper on the chute to prevent bridges in the hopper. A typical stirring device is, as illustrated in FIGS. 20 and 21, a horizontal agitator that has a stirring part (agitator) 54b for vertically agitating powder/granular material with respect to a shaft 54a protruding from the side of the chute 53. Furthermore, as illustrated in FIG. 25, a table-type powder/granular material feeding device is available that rotates powder/granular material in a circumferential direction by means of a stirring part 72 that is called a vertical agitator. The stirring part 72 rotates about a shaft 71d protruding upward from a bottom 71a of a chute 71. Moreover, as illustrated in FIGS. 22, 23A, and 23B, a powder/granular material feeding device of flexible chute type is available in which a chute 81 disposed under the hopper 82 is made of flexible resin and is deformed from the outside so as to move an internal material.
The stirring device (stirring part) also supplies powder/granular material into an outlet portion provided under the stirring device and stabilizes the powder/granular material with a high filling rate in the outlet portion. The stable filling rate in the outlet portion remarkably contributes to more stable discharging capability and higher feeding accuracy (small variations).
The outlet portion is a screw-type outlet (double screws or a single screw) using the screws 52a and a screw 84 illustrated in FIGS. 21 to 23B. The outlet portion may be vibration type, belt type, disk type, and so on. However, the screw type is the most suitable for powder/granular materials having various properties, e.g., adhesion and discharge. The single screw 84 (see FIGS. 22, 23A, and 23B) is preferably used for a less adhesive material, whereas the double screws 52a (see FIG. 21) capable of self-cleaning is preferably used for a highly adhesive material. The double screws capable of offsetting screw pulsations are frequently used for high feeding accuracy.
Hence, the stirring part of the stirring device disposed in the chute has the function of feeding a material to the screw acting as the outlet portion under the hopper as well as the function of preventing bridges in the hopper disposed on the chute. The stirring part is a mechanism that plays a key role in the powder/granular material feeding device and is quite important in determining the performance and character of the powder/granular material feeding device.
The conventionally used powder/granular material feeding devices of the respective types and the advantages and disadvantages thereof will be specifically described below.
FIGS. 20, 21, and 26 illustrate the powder/granular material feeding devices, each having the horizontal stirring part called a horizontal agitator. The two types of powder/granular material feeding devices have the dome-shaped chute 53 in FIGS. 20 and 21 and a square chute 91 in FIG. 26. The rectangular-prism chute 91 in FIG. 26 can be produced by boiler making (also called a welded structure). A screw 92a with a flat driving side and a flat discharging side allows the opening of the chute 91 to have a relatively simple sealing structure that advantageously facilitates sealing. Moreover, the rectangular-prism chute 91 and a screw casing 92b that accommodates the screws (outlet portion) 92a are linearly in contact with each other, forming a long area of feeding to the screws 92a. Thus, the powder/granular material feeding device including the rectangular-prism chute 91 can feed a material that is hard to come into the screws 92a, for example, a film material.
However, a powder/granular material feeding device 90 disadvantageously requires welding over the corners of the chute 91, is likely to deposit or leave a material on the corners of the chute 91, and increases a screw length as compared with the dome-shaped chute 53 (see FIG. 21), forming a dual support structure. Moreover, a hopper (not shown) disposed on the rectangular-prism chute 91 is inevitably shaped like a rectangular prism, so that the chute 91 and the hopper need to be joined with clamps at several points. Disadvantageously, the attachment and detachment of the hopper requires considerable effort and time as compared with the dome-shaped chute. Reference numeral 93 in FIG. 26 denotes the stirring part of the horizontal agitator.
The powder/granular material feeding device with the dome-shaped chute 53 in FIGS. 20 and 21 has no corners on the chute 53, which reduces powder/granular material deposition and residue and facilitates cleaning. The cylindrical hopper 51 that hardly allows powder/granular material deposition is attached on the upper part of the chute 53, and a clamp band 56 can be used on a joining area between the chute 53 and the hopper 51. Advantageously, the hopper 51 can be attached and detached with less effort and time.
However, the dome-shaped chute 53 provided in the powder/granular material feeding device disadvantageously needs to be manufactured with a mold, for example, by drawing or a lost-wax process, so that the dome-shaped chute 53 and a screw casing 52b that accommodates the screws 52a are joined with a slightly complicated configuration. Furthermore, the stirring part (agitator) 54b approaches closest to the screws 52a only at a center point. A space is formed between an agitation range and the screws 52a, and powder/granular material is fed to the screws 52a with a shorter feeding area as compared with the rectangular-prism chute, so that the powder/granular material is less supplied to the screws 52a than in the rectangular-prism chute.
The powder/granular material feeding device having the horizontal agitator vertically agitates powder/granular material by means of the stirring part 54b or 93, vertically moving the powder/granular material regardless of whether the chute is a rectangular prism or a dome. Thus, in the case where the amount of discharged powder/granular material is controlled during measurement, the weight is likely to fluctuate. Particularly, the powder/granular material feeding device is seriously affected by fluctuations in weight during an operation with a low flow rate and thus a control state may be disturbed by the powder/granular material feeding device, disadvantageously leading to lower feeding accuracy.
FIG. 25 is a perspective view illustrating the table-type powder/granular material feeding device. In such a powder/granular material feeding device, the bottom 71a of the chute 71 is shaped like a round table. As has been discussed, powder/granular material is agitated in the circumferential direction by the stirring part 72 that is called a vertical agitator. The stirring part 72 rotates about the shaft 71d that protrudes upward from the bottom 71a of the chute 71. Reference numeral 71b in FIG. 25 denotes a chute outlet for feeding powder/granular material to an outlet portion from the chute 71. In such a so called table-type powder/granular material feeding device, the bottom 71a of the chute 71 can have a relatively large size, allowing a hopper (not shown) to have a large base area. Since the stirring part 72 including the vertical agitator can be provided, bridges can be effectively prevented. A joining area between the chute 71 and the hopper is inevitably circular, allowing the use of a clamp band similar to the clamp band 56 in FIG. 21.
Even in the case of a granular material, e.g., a pellet that does not need agitation, the simple stirring part (agitator) 72 is necessary to prevent quite a large amount of the material from remaining on the bottom 71a shaped like a flat table. Also in the case of a powder material, the material is deposited and left in a gap between the stirring part 72 and the surface of the bottom 71a and on the blade of the stirring part 72. A screw 75 disposed directly under the bottom (table surface) 71a is laterally shifted so as to avoid a driving shaft that rotates the stirring part 72, limiting an area of feeding to the screw 75. Moreover, the stirring part 72 only passes above the screw, reducing the capability of filling to the screw 75.
The screw 75 can be centered by providing another room under the bottom (table surface) 71a. Moreover, the provision of a stirring part (horizontal agitator) 73 around the screw 75 can improve the capability of filling to the screw 75, though a large amount of powder/granular material is left under the screw 75.
FIGS. 22, 23A, and 23B are a perspective view and side cross-sectional views of a powder/granular material feeding device 80 that includes the hopper 82 shaped like a square cylinder (that is, a rectangular prism) on the flexible chute 81. The side of the chute is deformed by a paddle 83 from the outside of the flexible chute 81 to move internal powder/granular material, thereby preventing bridges. In the case of the rectangular-prism hopper 82, powder/granular material having medium flowability, e.g., calcium carbonate or talc is highly unlikely to cause bridges. The rectangular-prism hopper 82 can be more easily manufactured than a conical or pyramidal hopper in terms of the manufacturing cost, leading to lower manufacturing cost. Furthermore, the flexible chute type has a lower resistance than in the rotations of a stirring part acting as a rotating object in powder/granular material. Thus, the capacity of a motor for driving the stirring part can be reduced, achieving a clear cost advantage. Since the resistance is low, the flexible chute 81 can have a larger size, so that the hopper 82 can advantageously have a large capacity with relative ease.
In the powder/granular material feeding device 80 including the flexible chute 81, however, the flexible chute 81 is a consumable item that is an elastic body made of resin or the like. Thus, the flexible chute 81 needs replacing every several years (e.g., two years), requiring running cost for each replacement of the flexible chute 81. Since the paddle 83 desirably comes into contact with a flat surface, the flexible chute 81 and the hopper 82 have squares, which may lead to deposition of a material on the corners of the hopper 82 so as to cause bridges. In other words, the capability of preventing bridges is slightly lower than in the case where the hopper 82 has a circle. Moreover, agitation by the paddle 83 does not generate a force pressing a material to the screw 84, so that the material is dropped and inserted to the screw 84 only by its own weight. Thus, the capability of filling a material to the screw 84 is lower than in internal agitation. Since the flexible chute 81 is vibrated by swinging of the paddle 83, the weight is likely to fluctuate. In the case where the amount of discharged powder/granular material is controlled during measurement, the feeding accuracy may decrease.